Mark Wahlberg's "Pain & Gain" role required the actor to go through a transformation, gaining pounds of muscle to play the bodybuilding part. In the film, some bodybuilders kidnap one of their clients to get a quick payday, which required Wahlberg as an actor to rely heavily on his criminal past.

Mark Wahlberg spoke about "Pain & Gain," revealing that his own interest in bodybuilding helped fuel the need to become bigger and bulkier to play the part.

"I had a huge preparation," Mark told Access Hollywood during the movie premiere Thursday. "And then also just letting loose, becoming this character. I was always fascinated with the world of bodybuilding."

But bodybuilding wasn't the only element to the film. "Pain & Gain" has a strong criminal component to it, even though the characters are inexperienced at kidnapping. Wahlberg had to draw on his own experiences— over 20 run-ins with the Boston police department as a youth, an assault conviction, and 45 days in jail— to make the movie seem more realistic.

When Wahlberg's character kidnaps a client to make quick cash, the actor suggested to director Michael Bay that he put on an accent to disguise his voice. The 41-year-old actor pointed out that robbing someone familiar would pose problems if the mark recognized the voice robbing him. Fortunately, Wahlberg already had a voice picked out— an acquaintance from Panama.

"I've been training [the mark in the movie] for quite some time, so I had to disguise my voice," he told MTV News.

"[My co-stars Dwayne "The Rock" Johnson and Anthony Mackie] don't have that criminal mentality like me," Wahlberg continued. "I know how to do s---. I've robbed people in the past, so you can't be just like, 'Hey, dude, what's up? It's me, Mark. Listen— give me your s---!"

For Johnson, his past as a professional wrestler and his parts in movies helped him be prepared to pack on the muscle for the role in advance.

"I was really fortunate because I was coming off of 'G.I. Joe,' I was coming off of 'Fast & Furious' at that time too," he told Access Hollywood. "A lot of those projects supported and fostered the type of training I was doing."

"The biggest departure was the vulnerability … and playing a character who is easily influenced and who's just out of prison and looking for salvation," Johnson explained.